Respiration is a metabolic process common to all living things. Here, you will learn the definition, location, processes, and formula for cellular respiration. At the end, you can test your knowledge with a short quiz.

What is Respiration?

When you hear the word 'respire,' you probably think of breathing. When you breathe, you are taking in oxygen with each inhale and releasing carbon dioxide with each exhale. This gas exchange is important for respiration, but while breathing is a physical process, respiration can be thought of as more of a chemical process. All organisms, from a single bacterial cell to a coral reef colony to a blue whale, undergo respiration.

Food molecules absorbed after digestion are taken in, broken down, and the energy freed in the process is used to power the organism's movements and physiological functioning. Respiration is the biochemical process in which the cells of an organism obtain energy by combining oxygen and glucose, resulting in the release of carbon dioxide, water, and ATP (the currency of energy in cells).

When we examine the equation for cellular respiration, we see that the reactants are glucose and oxygen (for aerobic respiration), and the products are carbon dioxide, water, and ATP. Note the number of oxygen, carbon dioxide, and water molecules involved in each 'turn' of the process.

Balance with Photosynthesis

Respiration is the antithesis to the process of photosynthesis, in which carbon dioxide and water are taken in by autotrophs, along with sunlight, to make glucose and oxygen. Autotrophs include any photosynthesizing organisms, such as plants and algae, all of whom also undergo respiration. The products of photosynthesis are taken in by heterotrophs, organisms who cannot make their own energy and rely upon autotrophs for food. The byproducts of their respiration - carbon dioxide and water - are then used for photosynthesis. In a balanced ecosystem, this exchange is an example of a negative feedback loop.

The balance of photosynthesis (green) and respiration (red)

Three Phases of Respiration

In prokaryotic cells, respiration takes place in the cytosol and across the cell plasma membrane. In eukaryotic cells, it occurs in the cytosol and in the mitochondria. Mitochondria are the powerhouses of eukaryotic cells, and contain high surface areas of membrane folds on which respiration activity can be maximized.

Respiration occurs in a similar way to the internal combustion of your car engine: organic compounds and oxygen go in, carbon dioxide and water come out, and the energy released in the process powers the car or cell. Respiration and combustion are both exergonic processes, in which energy is released from the breaking of molecular bonds. To crank out ATP from the breaking of glucose bonds, respiration occurs in three phases:

Glycolysis: The original glucose molecule (from food) is broken down into pyruvic acid, which is oxidized into CO2 and water, leaving a two-carbon molecule called acetyl-CoA. Two ATP are generated in this process. This occurs in the cytosol.

The Citric Acid Cycle: The acetyl-CoA from glycolysis is added to an existing carbon chain and sequentially broken down, releasing more CO2 (byproduct) and releasing electrons, which are added to the acceptor molecules. Two ATP are generated for each turn of this cycle. This occurs in the mitochondrial matrix.

Oxidative Phosphorylation: The electron acceptor molecules drop off the electrons, which work to pump H+ ions in high concentration on one side of the plasma membrane, creating a gradient pressure that churns the ATP synthase enzyme, generating about 32 ATP. The remaining electrons are taken by oxygen, which then combines with free hydrogens to create water. Think of the enzymes like a water mill, with water pressure moving the mill, and kinetic energy generating electricity.

A mitochondrion and the stages of cell respiration

In a nutshell, breaking the glucose molecule and subsequent carbon chains during the citric acid cycle releases free electrons to be used in firing up the electron transport chain, thus powering the ATP synthase enzyme to crank out ATP.

Anaerobic Respiration

Many organisms can still metabolize even if there is no oxygen present! Respiration can be anaerobic if no oxygen is taken in. In this case, an alternative electron acceptor, such as zinc or manganese, can be used to carry out the reaction. Most prokaryotes undergo a form of anaerobic respiration and some eukaryotic processes are anaerobic.

Two examples of eukaryotic anaerobic respiration include lactic acid fermentation and alcohol fermentation, named for their by-products. Lactic acid fermentation is the process in your muscle cells commonly associated with soreness after intense exercise; your muscles contract too rapidly for aerobic respiration, so an anaerobic pathway temporarily takes over. Anaerobic respiration is typically less efficient in ATP production than aerobic respiration.

Types of anaerobic respiration pathways

Lesson Summary

Respiration is the biochemical process in which the cells of an organism obtain energy. It turns the food you eat into the energy that your body needs to function. It is the most basic level of metabolism in all living things, and the overall equation for cellular respiration says that glucose plus six oxygen molecules react to form six carbon dioxide molecules plus six water molecules plus ATP (or energy).

There are three phases of respiration: glycolysis, the citric acid cycle, and oxidative phosphorylation. These metabolic processes break down glucose and subsequent products to provide ATP. Many organisms can undergo anaerobic respiration in the absence of oxygen. Cellular respiration is complex in its components, yet simple in its function.

Learning Outcomes

When you have finished this lesson on respiration, find out whether you're able to:

Provide the definition of respiration

Compare respiration to photosynthesis

Enumerate the three phases of the respiration process

Recall the purpose of respiration and its overall equation

Recognize the way in which cellular respiration can happen without oxygen

Summary:

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